It seems plausible that she felt a pain from a burn that happened at some time
before that, perhaps hours or even days, so it's quite likely that she did not notice it at the time the burn happened. That lag makes it easier to forget an incident to which no thought was given at the time; I do it when I trip on something, say "ouch", continue on my way, then wonder later where that bruise came from. The best we can tell you is that it looks exactly like the kind of burn pattern for which we have seen many examples from the grid on a hairdryer (many of them on people with no memory of the burn being inflicted) and I don't think any other plausible causes have been suggested.
edit to add: It even appears to be pattern number one from Jacques Vallee's chart that was posted by
@Mick West
View attachment 56490